Not able to populate the values from dropdown

Actions & Logic

I want to create a document in a collection called "procedure" and a field is ref to a collection called "department" .
Now i have a form in a component and i populated the dropdown values with Doc Ref from collection "Departments" and on submit button when i tried to add the dropdown as ref its not adding. There are so many confusing threads to smilar questions, but as a biginner i cannot understand them, can anyone please give a clear solution. ๐Ÿ™

here:

What have you tried so far?

I tried creating a component variable but i cannot select the value of the selected dropdown value

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